The most severe form of hypothyroidism is:

A) Cushing's disease.
B) Addison's disease.
C) myxedema.
D) Graves disease.

Myxedema is the most severe form of hypothyroidism and is characterized by severe mental and physical sluggishness, weight gain, and puffiness of the face and hands. Cushing's disease is a form of hyperadrenocorticism, Addison's disease is a form of adrenal insufficiency, and Graves disease is a form of hyperthyroidism.